1. Field of the Invention
This invention relates to the alignment of ferro electric liquid crystal display (FLCD) devices.
2. Discussion of Prior Art
Such devices are formed by a thin layer of a chiral smectic liquid crystal material contained between two cell walls. The walls carry electrode structures, e.g. row and column electrodes forming an x,y matrix of addressable pixels. Electrical voltages are applied to the row and column electrodes in sequence so that all pixels are addressed in sequence. Prior to assembly, one or both of the inner faces of the cell wall are treated to an alignment treatment that provides both alignment direction and an amount of surface tilt to contacting liquid crystal material. Typical chiral tilted smectic materials have the following phases with decreasing temperature:--isotropic-cholesteric-smectic A-S*, where the asterisk denotes chirality and the smectic phase is tilted smectic such as smectic C, smectic I, etc.
One known type of FLCD is a surface stabilised (SSFLCD) device which is a bistable device. Switching between its two stable states is by application of a dc pulse of appropriate sign, amplitude, and time duration. Smectic liquid crystal molecules can be envisaged as rotating around the surface of a cone as they switch; the cone angle varies with material. Some devices show a minimum in their voltage vs. time switching characteristics (so called .tau.v minimum). Such a device and a typical addressing scheme is described in GB-2,232,802.
It is known that FLCD can show a patchy appearance due to the smectic liquid crystal materials forming into a mixture of two different micro layer arrangements known as the C.sub.1 and C.sub.2 states (ref. J. Kanbe et al Ferroelectrics (1991) vol 114, pp 3). These micro layers are chevron shape and the angle between cell wall and micro layer tilt angle can be indicated by .delta.. Both C.sub.1 and C.sub.2 can form as the material cools down from an isotropic state during manufacture; boundaries between these two states may be indicated by the known zigzag defect. Also, the two different states may form in a completed device, due to shock, e.g. mechanical damage when dropped or by application of too high an electric field etc.
Devices with a single state are required; preferably alignment in the C.sub.2 state is obtained since this allows a faster switching at lower voltages. It has been a problem for several years to provide a clear, defect free device. Existing solutions include devices where only the C.sub.1 state occurs as in U.S. Pat. No. 5,543,943 Hanyu et al. It is also known to use a high surface tilt .xi. GB-2,210469, U.S. Pat. No. 4,977,264, to provide a device free of zigzag defects. GB-A-2,274,519 describes selection of a C.sub.2 state by a surface pretilt .xi. equal to or greater than 10.degree., a chevron angle .xi. in the range 2.degree. to 15.degree., a one half chiral smectic cone angle .theta..sub.c, such that .theta..sub.c -.delta..gtoreq..xi..
Good and consistent alignment is necessary for good devices. There are several ways of providing alignment. For example the walls may be coated with a polymer then unidirectionally rubbed, silicon oxide may be evaporated on to the cell (the so called oblique evaporation technique), and grating surfaces (formed by embossing or by photolithographic techniques). Such alignment treatment both give an alignment direction and a surface tilt to liquid crystal molecules in contact with the treated surface; this may be termed a surface alignment induced pretilt and given the symbol .xi.; conventionally the pretilt is measured in a nematic phase. It is known to use long pitch in a cholesteric phase above the smectic phase e.g. GB-2,209,610, U.S. Pat. No. 5,061,047, GB-2,210,468B to improve alignment in the chiral smectic phase.